Eukaryotic cell ultrastructure

• Nucleus — bounded by a double membrane (nuclear envelope) with nuclear pores that let mRNA out. Contains chromatin.
• Nucleolus — makes ribosomal RNA and assembles ribosomes.
• Rough ER — flattened membrane sacs studded with ribosomes; site of synthesis and transport of proteins destined for secretion.
• Smooth ER — no ribosomes; makes lipids and steroids.
• Golgi apparatus — modifies proteins (e.g. adds carbohydrate to make glycoproteins), packages them into vesicles and makes lysosomes.
• Mitochondrion — double membrane; the inner one folded into cristae bearing ATP synthase; the matrix holds the Krebs-cycle enzymes.
• Lysosome — vesicle of hydrolytic enzymes.
• Centrioles — organise the spindle in animal cells.

Resolution vs magnification. A light microscope resolves ~200 nm; a transmission electron microscope resolves ~0.1 nm because the wavelength of an electron beam is far shorter than that of light. That is why ultrastructure needs an EM — magnification alone would just give a bigger blur.

Your turn — magnification

Your turn — actual size

The protein secretion pathway

Trace an extracellular enzyme from gene to outside the cell:

• The gene is transcribed in the nucleus; mRNA leaves through a nuclear pore.
• The mRNA is translated on a ribosome bound to the rough ER; the polypeptide enters the ER lumen and begins to fold.
• A vesicle buds off the rER and fuses with the Golgi apparatus.
• The Golgi modifies the protein (e.g. glycosylation) and packages it into a secretory vesicle.
• The vesicle moves along the cytoskeleton and fuses with the cell-surface membrane: exocytosis (ATP-requiring).

Organelle → job

Tap a job on the left, then the organelle that does it.

Prokaryotic ultrastructure

A prokaryote has no nucleus and no membrane-bound organelles. It does have:

• a single circular DNA molecule, free in the cytoplasm (the nucleoid), and often plasmids;
• 70S ribosomes (smaller than the eukaryotic 80S — which is why some antibiotics can target bacterial ribosomes without harming ours);
• a cell wall of peptidoglycan (murein), sometimes a protective capsule, and often a flagellum for movement;
• mesosomes — infoldings of the membrane — and, of course, a cell-surface membrane and cytoplasm.

Prokaryote, eukaryote, or both?

Tap a feature, then tap the cell type it belongs to.

Gametes and fertilisation

Sperm: an acrosome full of hydrolytic (protease) enzymes; many mitochondria in the midpiece to supply ATP for the flagellum; a haploid nucleus and very little cytoplasm.
Egg: large, packed with food reserves; a zona pellucida (glycoprotein coat); cortical granules just beneath the membrane.

Fertilisation in mammals:

• The sperm binds the zona pellucida; the acrosome reaction releases enzymes that digest a path through it.
• The sperm and egg membranes fuse; the sperm nucleus enters.
• The cortical reaction: cortical granules fuse with the egg membrane and their contents thicken the zona pellucida, making it impenetrable — this prevents polyspermy.
• The two haploid nuclei fuse: a diploid zygote.

The cell cycle and mitosis

Interphase is by far the longest stage: G1 (growth, organelles replicated), S (DNA replicated — each chromosome becomes two sister chromatids), G2 (growth and checking). Then mitosis, then cytokinesis.

• Prophase — chromosomes condense and become visible; nuclear envelope breaks down; spindle forms.
• Metaphase — chromosomes line up on the equator, attached to spindle fibres by their centromeres.
• Anaphase — centromeres divide; sister chromatids are pulled to opposite poles (an ATP-requiring process).
• Telophase — chromosomes decondense; two nuclear envelopes re-form.

Core practical 5: prepare and stain a root tip squash to observe the stages of mitosis, then calculate the mitotic index.

Your turn — mitotic index

Meiosis and the sources of variation

Meiosis has two divisions and produces four haploid, genetically different cells. Two mechanisms generate that variation:

• Crossing over (prophase I) — homologous chromosomes pair up as bivalents; chiasmata form and equivalent sections of non-sister chromatids are exchanged, creating new combinations of alleles on a chromosome.
• Independent assortment (metaphase I) — each homologous pair lines up on the equator independently of every other pair. For n pairs there are 2ⁿ possible combinations in the gametes.

Add random fertilisation and the number of possible offspring genotypes becomes astronomical.

Loci, linkage and sex linkage: a locus is the position of a gene on a chromosome. Genes on the same chromosome are linked and tend to be inherited together (only crossing over separates them), so they do not assort independently. Genes on the X chromosome are sex-linked: males (XY) have only one copy, so a recessive X-linked allele (e.g. haemophilia) is always expressed in them.

Your turn — independent assortment

Mitosis or meiosis?

Stem cells and differential gene expression

A stem cell is undifferentiated and can divide repeatedly.

• Totipotent — can form any cell type plus the placenta (the zygote and cells of the very early embryo).
• Pluripotent — can form any cell type of the body, but not the placenta (embryonic stem cells).
• Multipotent — a limited range (adult stem cells, e.g. bone marrow → blood cells).

Every body cell contains the same genome. Cells differ because of differential gene expression: only some genes are transcribed in any one cell. Transcription factors bind to the promoter region of a gene and switch it on (or off), so a liver cell and a neurone read different parts of the same instruction manual.

The ethics are contested, and SNAB wants you to argue, not list: embryonic stem cells are the most useful but their extraction destroys an embryo; induced pluripotent stem cells (iPSCs) sidestep that but carry a risk of tumour formation. Society uses regulation (e.g. the 14-day rule) to balance the benefits against the objections.

Epigenetics and phenotype

Phenotype = genotype × environment. Height, skin colour and body mass are polygenic (many genes, each of small effect) and strongly environment-dependent, giving continuous variation.

Epigenetic changes alter gene expression without changing the DNA base sequence:

• DNA methylation — methyl groups added to cytosine in a promoter region prevent transcription: the gene is switched off.
• Histone modification — acetylation of histones loosens the DNA’s grip on them, making the gene more accessible to RNA polymerase: switched on. Deacetylation condenses the chromatin and switches it off.

Environmental factors (diet, stress, toxins) can add or remove these marks, and the marks can be copied and passed on when the cell divides — and in some cases across generations.

Switching a gene off

Cells → tissues → organs → systems

Differentiated cells are organised into a hierarchy:

• Tissue — a group of similar cells with a shared function (e.g. squamous epithelium, cardiac muscle).
• Organ — several tissues working together (the heart contains cardiac muscle, connective tissue, epithelium and nervous tissue).
• Organ system — several organs (the circulatory system: heart, arteries, veins, capillaries).

Each level of organisation exists because it allows division of labour: specialised structures do one job extremely well, which a single generalist cell never could.

The big ideas to know

You have covered the whole of SNAB Topic 3. Press Finish to see your score.